1. Field of the Invention
This invention relates generally to circuits and, more specifically, the present invention relates to circuits including series stacked capacitors.
2. Background Information
One function of a power converter is to convert rectified alternating current (AC) power into a regulated direct current (DC) output. FIG. 1 shows elements included at an input 103 to a power converter 101. Diode bridge BR1105 rectifies AC input. Series stacked capacitors C1107 and C2109 are coupled across diode bridge BR1105 to smooth the output voltage of diode bridge BR1105. Series stacked capacitors are common in power supplies that can be configured to operate using multiple different input voltages such as for example either 115 VAC or 230 VAC. As shown, switch SW1111 is coupled between diode bridge BR1105 and the connection point 113 between series stacked capacitors C1107 and C2109. When operating for example at 230 VAC, switch SW1111 is opened. When operating for example at 115 VAC, switch SW1111 is closed.
Resistors R1115 and R2117 are coupled across series stacked capacitors C1107 and C2109 as shown to maintain roughly equal voltages across C1107 and C2109 and provide the necessary bleed current needed to balance the voltage across series stacked capacitors C1107 and C2109 when the supply is configured for 230 VAC input. During 115 VAC operation, the two series stacked capacitors C1107 and C2109 function as part of an input voltage doubler circuit. When configured for 230 VAC, however, the two series stacked capacitors C1107 and C2109 have no DC connection to a center point voltage other than that which is provided by resistors R1115 and R2117. Without resistors R1115 and R2117, the center point voltage of the two series stacked capacitors C1107 and C2109 can deviate from the ideal xc2xd DC input due to capacitor leakage current, which can cause one capacitor to have more voltage stress than the other capacitor. In fact, it is possible that one of the capacitors can be over-voltage stressed and become damaged.
Resistors R1115 and R2117 therefore provide a solution to the balance problem by providing bleed current. The resistance values of R1115 and R2117 must be low enough to establish a bleed current that is several times higher than the worst case leakage current imbalance between the series stacked capacitors C1107 and C2109 in order to be effective. This requires the resistors to dissipate much more power than the actual power dissipated as a result the difference current between the two capacitors. Consequently, resistors R1115 and R2117 result in significant input power consumption with respect to many standby or output no-load requirements of a power supply converter coupled to receive the rectified AC power.
An active circuit that substantially reduces the bleed current required for balancing leakage current in series stacked capacitors is disclosed. This active circuit can also be designed to disconnect any bleed current when capacitors are configured for voltage doubler operation as bleed current is not necessary in this configuration. In one embodiment, the circuit is switched across a capacitor to provide bleed current as required to balance the leakage current. In one embodiment, the voltage at the connection point between two capacitors is bounded within a few volts of two reference voltages. In one embodiment, the bleed current is substantially equal to the difference in leakage current between two series stacked capacitors. In one embodiment, the active circuit includes a sink-source follower circuit. In one embodiment, a sink-source follower circuit includes inputs where each are referenced to voltages that are offset by a fraction of the voltage applied across the series stacked capacitors and the outputs of the sink-source follower circuits are coupled to the connection point between two series stacked capacitors. In one embodiment the offset is limited to a very low value and can be substantially zero. In one embodiment, the sink follower circuit includes a PNP bipolar transistor coupled to a source follower circuit, which includes an NPN bipolar transistor. In one embodiment, resistors are connected in series with the collector of each of the bipolar transistors to limit the peak current conducted by the bipolar transistors. In one embodiment, the active circuit is used in a power supply circuit
In another embodiment, a method of substantially reducing the bleed current required for balancing leakage current in series stacked capacitors utilizing an active circuit is disclosed. In one embodiment, the bleed current required is switched in as required to balance leakage current from the series stacked capacitors. In one embodiment, the bleed current from the active circuit is switched off and is substantially equal to zero when series stacked capacitors are configured for doubler operation. In one embodiment, the bleed current is substantially equal to a difference in leakage currents between two capacitors. In one embodiment, the active circuit includes a means to maintain an output voltage, connected to the connection point between two series stacked capacitors, relative to an input reference voltage. In one embodiment, the disclosed method is applied in a power supply circuit. Additional features and benefits of the present invention will become apparent from the detailed description and figures set forth below.